1. Field of the Invention
The present invention relates to an ignition coil which generates a voltage applied to an ignition plug for an internal combustion engine.
2. Description of Related Art
There is conventionally known an ignition coil in which a secondary coil arranged at an outer peripheral side of a primary coil is increased in voltage through mutual induction with the primary coil to generate an applied voltage to an ignition plug. There is proposed an ignition coil as one kind of such an ignition coil in which an outer periphery core is arranged in opposition to an outer peripheral surface of the secondary coil and a plastic member is interposed between the secondary coil and the outer periphery core to realize electrical insulation (for example, JP2005-50892A).
In JP2005-50892A, since outer edges square-built on an inner peripheral surface of the outer periphery core exactly face an outer peripheral surface of the secondary coil, an electrical field generated between the secondary coil and the outer periphery core tends to easily concentrate on the above outer edge. Such local concentration of the electrical field causes a treeing phenomenon in the plastic member between the secondary coil and the outer periphery core to degrade the plastic member. As a result, further degradation of the plastic member produces dielectric breakdown between the outer peripheral surface of the secondary coil and the outer edge on the inner peripheral surface of the outer periphery core. Therefore, a lifetime due to the dielectric breakdown is shortened, raising the problem with durability of the ignition coil.